As is well known in the field of oil and gas prospecting, certain geological formations are more likely to contain hydrocarbon products (i.e., oil and natural gas) than others; the field of seismic surveying regions of the earth in prospecting for oil and gas reservoirs is thus directed to the identification of these product-bearing formations. The presence or absence of oil and gas in specific formations generally depends, of course, upon the geological history of the particular region of the earth. Significant hydrocarbon reservoirs not only require adequate porosity, permeability, and thickness on the part of the rock itself so that the formation can retain the hydrocarbon product, but also must have had a geologic history that involved the generation and migration of hydrocarbons from a source rock, as well as trapping, sealing and preservation of the hydrocarbons in the reservoir rock.
In North America, one type of geological formation that is well known to generally bear oil and gas is Morrow sandstone, also referred to as Morrow sand. Because of their geologic history, and also because of the permeability and porosity of the sandstone structure, Morrow sands typically contain hydrocarbon products in useful amounts. However, since Morrow sands in North America typically correspond to ancient river and stream beds, and as such are generally in the form of channels, bars, and other discontinuous sand bodies at varying depths in the earth, rather than strata of large area, it is difficult to find Morrow sands in typical seismic surveys. This difficulty in finding Morrow sands is exacerbated by the acoustic similarity of Morrow sands to neighboring limestones and shales. As such, seismic reflections from Morrow sands generally do not stand out from these neighboring structures in conventional seismic surveys, either in amplitude or in character.
By way of further background, a known technique useful in conventional seismic surveys is amplitude-versus-offset ("AVO") analysis. According to the AVO approach, information regarding a subsurface interface is obtained not only from the stacked normal-incidence amplitude of seismic energy, but also from the behavior of the detected seismic reflections as a function of the angle of incidence from the normal. Dependence of the reflected amplitude upon the angle of incidence is believed to be due to the transformation of incident pressure waves to reflected shear waves at certain interfaces, thus reducing the amplitude of reflected pressure waves, such transformation being angularly dependent. The extent to which pressure waves are transformed into shear waves, as a function of angle of incidence, is due to differences in acoustic impedance between the formations on either side of the reflective interface.
According to conventional AVO analysis, one considers the amplitude R of a reflected seismic wave from an interface (i.e., the "target horizon") as a function of the angle of incidence .theta. from the normal according to the following relationship: EQU R(.theta.)=A+B sin.sup.2 .theta.
In this case, A is the zero-offset response (also referred to as the AVO intercept), while B is the AVO slope or gradient since it is representative of the rate of change of amplitude with the square of the angle of incidence. A seismic trace, which is the time-dependent signal received at a geophone, may similarly be represented by a function S(t,.theta.) as follows: EQU S(t,.theta.)=A(t)+B(t)sin.sup.2 .theta.
Since AVO analysis is directed to determining the intercept and slope values at specific points in time along a trace (i.e., depth points), corresponding to a particular horizon, a conventional approach plots the A and B values at sampled times of the trace, thus eliminating the time-dependency from the analysis.
Theoretical values for A and B can be calculated for isolated rock interfaces (i.e., at specific horizons), through the use of the linearized Zoeppritz equations and based upon typical values for compressional velocity, density and Poisson's ratio for the strata on either side of the interface of interest, as described in Swan, "Properties of direct AVO hydrocarbon indicators", Offset-dependent reflectivity--Theory and Practice of AVO analysis (Castagna, J. P. & Backus, M. M., eds., Soc. Expl. Geophys., 1993), pp. 78-92. As described therein, variations in the A and B values for particular interfaces from the theoretical A-versus-B trend line for the expected stratigraphic sequences can indicate the location of interfaces in the survey.
It is therefore an object of the present invention to provide a method and system for analyzing conventional amplitude-versus-offset seismic survey data in such a way as to identify gas-bearing sandstone structures in the survey.
It is a further object of the present invention to provide such a method and system that can distinguish gas-bearing sands from neighboring limestones and shales.
It is a further object of the present invention to provide such a method and system that is particularly suitable for identification of Morrow sand geological structures.
It is a further object of the present invention to provide such a method and system that can operate upon conventional data using conventional computing equipment.
It is a further object of the present invention to provide such a method and system in which the reflection data taken by multiple detectors can be statistically normalized, so that coupling effects and other inaccuracies do not affect the analysis.
Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification together with the drawings.